It is standard to provide a motor-vehicle window, typically the rear one, with conductors through which electricity is passed to heat the glass panel, thereby eliminating condensation and ice. These conductors are normally provided as an array of horizontal and parallel lines of conductive paint applied to or imbedded in the window panel which itself can be of tempered glass or laminated safety glass. The ends of the parallel conductors are connected to vertically running bus connectors that are in turn connected to the on-board direct-current electrical system.
In recent times it has become common practice to incorporate the radio antenna in the windshield so as to make it vandal proof and to lower costs. In order to pick up vertically as well as horizontally polarized signals, at least two parallel but horizontally spaced vertical antenna conductors are provided that extend across and connect to several of the heater conductors and that are in turn connected to an antenna-output conductor. The radio-frequency output can be taken off the feed busses for the heater conductors also. Normally the vertical crosswise antenna conductors extend up past the array of horizontal heater conductors where they are connected to output conductors.
Such antenna systems often provide adequate reception, but their presence causes some degradation in performance of the window heater. In effect the crosswise antenna conductors provide low-resistance shunts for the current in the heater so that in the critical central region where the vertical antenna conductors are provided, heating is irregular, leaving uncleared spots on the windshield.